Since about 1978 rotational type CT scanners which can perform a scan of a single planar slice of a patient in as little as two seconds have been commercially available. The quest for speed in the data acquisition phase of medical CT scanners has been of substantial concern to designers and users of CT scanners for the obvious reasons of economy and image quality. Due to variations in the motions of various organs and anatomical functions in human beings, the influence of speed on image quality varies depending on the particular section of the patient being scanned. Thus, to provide added flexibility to the user, such modern CT scanners typically offer a choice of scan speeds, for example, 2, 4, and 8 seconds.
One popular configuration of these modern CT scanners is the so-called fourth generation system which includes a rotational source of radiation adapted to emit a fan beam of radiation typically directed at a patient supported within a scan circle, a stationary arc of uniformly spaced detectors about the center point of the scan circle, a data acquisition system and an image processor. In such scanners, the radiation source is rotated about the object to be imaged while emitting a beam of radiation of measured intensity. The array of detectors produce analog electrical signals proportional to the intensity of radiation incident on them. These signals are processed and sampled, typically once for every 96th of a degree of rotation of the source and subsequently reconstructed digitally into a planar image of the scanned slice.
It turns out that the temporal frequency content of a detector signal is determined not only by the spatial frequency content of the object under examination, but also by the angular velocity of the radiation source. Thus, under the controlled condition of constant dose, the same object will result in varying noise behavior for different scan speeds. See, for example, "CT Image Noise and Resolution Behavior in RC Filter-Based Projection Data Acquisition" by C. B. Lim, et al., in IEEE Transactions on Nuclear Science, Vol. NS-28, No. 1, February 1981, p. 152 et seq.
Regardless of the surce of the frequency content of the analog signal generated by the array of detectors, the high frequency content, essentially noise in the system, must be eliminated. Otherwise, when the signal is sampled, some of these high frequency components will appear impersonating low frequencies, a well known phenomena known as "aliasing". According to the sampling theorem, in a function having a finite limit in its rate of change, essentially all information may be recovered by fine sampling of a band limited portion of the function, Bracewell, R. N., 1965 The Fourier Transform and its Applications, New York, McGraw Hill, page 189 et seq. In prior CT scanning systems, the band limiting function was typically accomplished by filtering the analog detector signals by a fixed cut-off frequency analog filter where the cut-off frequency selection was dependent on the geometry of the specific CT system and the angular velocity of the scanner. Such designs are inherently limited in systems with either appreciable variations in the actual angular velocity relative to expected angular velocity of the source or in systems offering a choice of scan speeds. Since the cut-off filter has to be designed to accommodate the worst case situation, whatever the chosen cut-off frequency is, it can only approach optimum result for a single scan speed.